Spliced leader (SL) trans-splicing is an RNA processing event that forms the 5'termini of mature mRNAs by accurately joining as RNA small, separately transcribed exon (the SL) to the 5' end of pre-mRNAs. This form of splicing represents an evolutionarily important form of gene expression common in divergent groups of parasites of man. The applicant suggests that in vitro analysis of cis- and trans-splicing and polyadenylation will facilate the identification, role and interplay of important elements contributing to processing of transpliced pre-mRNA in metazoa.